Electric explosion initiator



Aug, 9, 1949. G. F. ROLLAND ELECTRIC EXPLOSION INITIATOR Filed March 5, 1945 iii v izi,:15:2:353:11.

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Guy F. Rolland,

INVENTOR BY 7 Z ATTOR EY Patented Aug. 9, i949 ELECTRIC EXPLOSION INITIATOR Guy F. Rolland, McKeansburg, Pa., assignor to Atlas Powder Company, Wilmington, Del., a

corporation of Delaware Application March a, 1945, Serial No. 580,856

9 Claims- ((1102-28) The present invention relates to electric explosion initiators.

An object of the present invention is an electric explosion initiator of improved resistance to premature firing by static electric discharges.

Another object of the present invention is an electric explosion initiator of a construction which permits easier and less.exacting manufacture.

A further object of the present invention is an improved insulating tube assembly for an electric explosion initiator.

Other objects will become apparent from the following description.

It has been recognized that electric explosion initiators may be susceptible to firing by the dis charge of static electricity between the shell and the firing circuit through the ignition composition.

As a means of overcoming static susceptibility, initiators have been provided in which part of the firing circuit lies closer to the shell wall than does any portion of the firing circuit which is covered by ignition composition. Then,- any spark discharging between shell and firing circuit should occur where these two elements are closest rather than through the ignition composition.

The static susceptibility of match type electric explosion initiators has been reduced in this manner by providing the match with wide tabs at a point above the locus of the ignition composition. The obtention of static protection by this means, however, requires careful centering of the match in the insulating tube which customarily surrounds it and also requires that the tube and match assembly be centered in the shell. Under conditions of commercial production, it is sometimes troublesome to insure proper centering of these elements.

According to the present invention, an initiator, of the above mentioned type, having a part of the firing circuit lying closer to the shell wall than does the portion of the firing circuit covered with ignition composition, is provided with an insulating tube having at least one conductive side surface, located inside said shell, and surrounding the portion of the firing circuit lying closer to the shell wall.

In match type initiators, the invention may be efiected by providing the conventionalinsulating tube with at least one conductive side surface.

The invention will be described in connection with the drawing, Figure 1 of which is a cross sectional view of a match type electric squib em- 2 bodying the invention, and Figure 2 of which is an enlarged view of a modified part of Figure 1.

The conventional metal shell is designated by H, I5 is an initiating charge loaded into the bottom of shell ii. I! is an insulating tube made of paper, plastic, rubber or other insulating material, and covered with conductive coating IS on its inside surface and with conductive coating 2i on its oustide surface. Tube ll surrounds electric match 23, all but the match head of which is embedded in insulating material 25, which may, for example, be a poured sulphur plug. Match 23 is made up of two metal plates (only the forward one of which appears in the drawing) separated by a piece of thin insulating material such as paper. It is to be noted that the wide or tab portion of the forward match plate very nearly touches the inside wall of insulating tube H. The rear match plate (not shown) is usually made narrower than the forward plate so as to prevent a short circuit between the plates in the event that the match touches the conductive insulating tube wall. At the lower edge of the match, a bridge wire connects the two plates and is covered by ignition composition so as to form a match head. Soldered to each of the match plates is a leg wire. In the drawing, one leg wire 21 covered with insulation 29 appears. Over the sulphur plug assembly are two poured waterproofing plugs, 3i and 33.

In a representative squib, the shell is made of aluminum 0.006" in thickness, and is 0.260" in inside diameter and 2" long. The insulating tube has an outside diameter of 0.255", a wall thickness of 0.015", and is {*g of an inch long. The tube is covered both on its inside and outside surfaces with aluminum foil. The match is 0.487" long and 0.035" in thickness. At the widest or tab portion it is 0.215" in width and at the narrow portion covered by ignition composi tion it is 0.090" in width.

While it is not intended to limit the invention by any particular-theory of operation, it may be pointed out that the resistance of electric explosion initiators to static electric discharges may be improved according to the present invention in a number of difierent ways.

Insulating tubes frequently are not of uniform dielectric strength throughout. Consequently, it often occurs that while the match head is physically farther away from the shell wall than is the tab part of the match from which discharge is desired, the dielectric properties of the tube are such that, were it not for the conductive coat ing, the shortest electrical path between the firin: circuit and the shell might lie through the match head. with the conductive coating present on the insulating tube, the current will always be carried around to the dielectricaliy weakest portion of the tube. Consequently, if the tab is physically closer to the insulating tube wall than the firing circuit in the match head, the spark will always jump as is desired from or to the tab. It may be noted that when the insulating tube fits snugly within the initiator shell this protection is provided when the inside only of the tube is conductively coated.

When there is considerable clearance between the insulating tube and shell wall the construc tion of the present invention also produces improved resistance to static electric discharges in another manner. In such a situation, while the match tab may be closer to the insulating tube wall than is the firing circuit in the match head, the tube itself may be cocked in the shell in such a position that the firing circuit in the match head is closer to the shell wall than is the match tab. However, since the outside foil will touch the shell wall, the current will be carried on the outside foil to a place adjacent the tab and the static electricity will discharge from or to the tab which, as stated, lies closest to the tube-walls, though not closest to the shell walls. It is to be noted that the improvement which is effected according to this method of operation -will take place whenonly the outside wall of the insulating tube is coated with conducting material.

It is further to be noted that when the tabs are of such size that there is a fair amount of clearance between them and the insulating tube walls, it is quite easily possible for the match head to get cocked in the tube so that the firing circuit in the match head is closer to the shell wall than are the match tabs even though the tabs may be closer to the insulating tube wall than is the firing circuit in the match head. Under these conditions the foil coated tubes additionally improve static resistance in the manner just described, except that the eifect occurs when only the inside tube wall is conductively coated.

Foil coated insulating tubes also often assist discharge of static electricity because of a peculiarity of their construction. The coated tubes are usually prepared in long lengths which are cut down to the proper size for use. The cutting operation tends to tear the foils and to curl them over the ends of the tube, This result is shown in Figure 2 which is an enlarged view of the top portion of a foil coated tube of the present invention. It will be noted that torn edges of inside foil l8 and outside foil 2i tend to meet on the end of tube IT. The approach of the two foils toward one another reduces the gap between the inside and outside foils and this reduction in gap contributes to static resistance.

As the discussion above indicates, some of the improved effects of the present invention are obtained when but one of the insulating tube.surfaces is coated with conducting material, but optimum results are obtained when both surfaces are made conductive.

It is not necessary that the insulating tube totally surround the match. It is sufficient that the wide or tab portion be surrounded. Similarly the conductive coating need only cover that part of the tube which surrounds the wide portion of the match.

Conductive materials, other than aluminum foil, may be employed as conducting coatings. For example, other metal foils are applicable, and

conductive paints or lacquerasuch as aluminum or bronze paints, may be used, Paints or lacquers are not generally as eifective as foils, for their conductivity is usually relatively low. However, the low conductivity of paints or lacquers permits their use as coating over the entire tube, for the slightly conductive circuit around the tube ends which they effect when used in this manner is not sufficient to produce the undesirable effects of a grounded firing circuit. Conductive paints and lacquers may also be used in addition to metal foils, particularly on the ends of the tubes.

The present surface conductive insulating tubes are also effective in diminishing danger due to static discharge in plastic shell detonators. While plastic shell detonators do not discharge static electricity between firing circuit and shell,

it has been found that static electricity will discharge from the firing circuit through the shell to a ground occurring outside the shell. The foil tube may assist such discharge to take place from or to the match tabs rather than the match head.

While the invention has been principally described in connection with initiators having metal match assemblies, it is to be understood that conductively coated tubes. may be used with similar effects in conjunction with match assemblies of other types, as for example, the foil type matches sometimes employed. Furthermore, conductively coated tubes may be used in static resistant electric explosion initiators of other types wherein a part of the firing circuit lies closer to the shell wall than does the portion or the firing circuit covered with ignition composition. It is only necessary that the tube surround the part oi. the firing circuit lying closer to the shell wall in the same manner as is done with the match type initiator.

Also, the invention is equally applicable to squibs, to detonators, and to electric explosion initiators containing various kinds of explosives or other materials to be initiated. I

What is claimed is: r

1. An electric explosion initiator including a shell and a firing circuit, a portion of which is covered with ignition composition and a part of which lies closer to the shell wall than that portion covered with ignition composition, wherein an electrically insulating tube surrounds said part and is coated with electrically conductive material on at least one side surface.

2. An electric explosion initiator, according to claim 1, wherein each side surface of said insulating tube is provided with an electrically conductive coating.

3. An electric explosion initiator including a shell, a firing circuit comprising an electric match, the width of which is greater at the tab portion than at the head portion, and an electrically insulating tube in said shell and surrounding said tab portion, wherein at least one side surface of said insulating tube is provided with an electrically conductive coating. v

,4. An electric explosion initiator, according to claim 3, in which each side surface of said insulating tube is provided with an electrically conductive coating.

5. An electric explosion initiator including a metal shell, a firing circuit comprising an electric match, the width of which is greater at the tab portion than at the head portion, and an electrically insulating tube in said shell and surrounding said match, wherein at least one side asraeaa surface of said insulating tube is provided with a metal foil coating.

6. An electric explosion initiator, according to claim 5, in which each side surface of said insulating tube is provided with a metal foil coating.

'7. An electric squib including a metal shell, 9. firing circuit comprising an electric match, the width of which" is greater at the tab portion than at the head portion, and an electrically insulatins tube in said shell and surroundin said match, wherein each side surface of said insulating tube is provided with a metal foil coating.

8. An insulating tube assembly for an electric explosion initiator. including, an electric match the width of which is greater at the tab portion than at the head portion and an electrically insulating tube surrounding said match, wherein at, least one side surface of said insulating tube is provided with an electrically conducive coating.

9. an insulating tube assembly according to at 6 claim 8 wherein each side surface 0! said insulating tube is provided with an electrically conducjtive coating.

GUY F. ROLLAND.

REFERENCES CITED The following references areof record in the tile of this patent:

UNITED STATES PA'I'ENTS.

Number Name Date 1,700,768 Kuechenm'eister Feb. 5, 1929 2,086,532 Burrows- July 13, 1937 2,086,548 Handforth July 13, 1937 15 2,105,674 Sosson Jan. 18, 1938 FOREIGN PATENTS Number Country Date 546,750 Germany Mar. 7, i932 Certificate of Correction Patent No. 2,478,838 August 9, 1949 GUY F. ROLLAND It is hereby certified that error appears in the printed specification of the above fiumbered patent requiring correction as follows:

Column 3, line 56, for the numeral 18 read 19;

and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 13th day of December, A. D. 1949.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.

Certificate of Correction Patent No. 2,478,838 August 9, 1949 GUY F. ROLLAND It is hereby certified that error appears in the printed specification of the above jumbered patent requiring correction as follows:

Column 3, line 56, for the numeral 18' read 19;

and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 13th day of December, A. D. 1949.

THOMAS F. MURPHY,

Assistant Oommissioner of Patents. c 

